1. Technical Field
The present invention relates to a sheet feeding apparatus for use in copying machines, printing machines, etc. for feeding paper, and more particularly to such an apparatus for feeding stacked sheets one by one separately and reliably.
2. Prior Art
Such sheet feeding apparatus can be divided generally into two types according to the method of separating stacked sheets individually. One is the type wherein sheets are separated by pawls, and the other type employs rollers for separation. Although simple in construction, the former type is somewhat instable in its sheet separating performance and is not suited for feeding sheets at a high speed. The latter type comprises a pair of rollers movable on the front and rear sides of a sheet in opposite directions to separate the sheet off and is generally superior to the former pawl type in separation efficiency.
The apparatus of the roller type include those wherein the rollers are spaced apart by a clearance (U.S. Pat. No. 4,208,046), and those wherein the rollers are in pressing contact with each other (U.S. Pat. No. 3,937,455). The apparatus of the clearance type can separate sheets satisfactorily insofar as the clearance between the rollers can be held constant for the sheets, but they have the drawback of involving difficulty in maintaining a constant roller-to-roller clearance and being complex in construction. On the other hand, the apparatus of the contact type tend to exhibit somewhat instable sheet separating performance since the pair of rollers are driven each at only one side of the roller shaft. Moreover they have the drawback of being complex in construction since the two rollers are held in pressing contact and driven individually. Further although the apparatus of the contact type are capable of separating sheets generally satisfactorily if suitable pressing means is used for the contact between the rollers, they are unable to handle sheets of widely varying thicknesses. They have another drawback in that for example when the pressing means is impaired in performance, the resulting variations in the sheet separating force lead to an instable operation.